4.1. Tasks, threads or processes

On most platforms, ECL supports native multithreading. That means
there can be several tasks executing lisp code on parallel and sharing
memory, variables and files. The interface for multitasking in ECL, like
those of most other implementations, is based on a set of functions and
types that resemble the multiprocessing capabilities of old Lisp Machines.

This backward compatibility is why tasks or threads are called
"processes". However, they should not be confused with operating system
processes, which are made of programs running in separate contexts and
without access to each other's memory.

The implementation of threads in ECL is purely native and based on
Posix Threads wherever avaiable. The use of native threads has
advantanges. For instance, they allow for non-blocking file operations,
so that while one task is reading a file, a different one is performing
a computation.

As mentioned above, tasks share the same memory, as well as the set of
open files and sockets. This manifests on two features. First of all,
different tasks can operate on the same lisp objects, reading and writing
their slots, or manipulating the same arrays. Second, while threads share
global variables, constants and function definitions they can also have
thread-local bindings to special variables that are not seen by other
tasks.

The fact that different tasks have access to the same set of data
allows both for flexibility and a greater risk. In order to control access
to different resources, ECL provides the user with locks, as explained in
the next section.